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Apple's Preview application has horrendous issues with some items - spot colors, transparency, overprints, etc. If you see strange behavior in Preview, the first thing you should do is check the PDF in Acrobat or Reader.

That triangle is constructed of several objects all using transparency (Blending modes). Flatten the transparency and it should look as expected in Preview.

If you need to correct the display of a PDF in Preview you have to "dummy down" the objects. Convert spot colors to RGB if possible, flatten transparency, remove overprints, etc.